The invention relates to methods and apparatus for the inspection of repeating patterns or designs on a moving web.
More specifically, the invention relates to inspection methods and apparatus wherein mirrors are mounted on a rotary drum of polygonal cross-section, a single mirror being mounted on each distinct outer side of the drum. Methods and apparatus of this type are described in U.S. Pat. No. 3,089,381 issued to Bogert.
In prior art rotary drum mirror methods and apparatus, the target web has customarily been positioned so that it is bisected by a line passing through the viewing point, i.e., the central point on the drum at which a viewing telescope is usually aimed. In prior art rotating mirror drum methods and apparatus, there were often provided 20 mirrors, one on each of 20 outer faces of the drum. In such systems, normally the web consisted of two straight sections which met to form an angle of approximately 162 degrees, said web angle facing and being aligned with the central viewing point of the 20 mirror rotating drum.
Although prior art mirror rotary drum web inspection systems have been in use for some time, they have not proved entirely satisfactory. One particularly serious problem associated with their operation has long been taken for granted. This is the problem known as the separation of image problem or the extraneous web image problem. In such systems, when one mirror and only one fills the viewing area at one instant, the image will appear to be steady and clear to the viewer. However, in commercial systems of the prior art, extraneous images appear to the viewer as a result of the rotation of the mirrors and the concomitant intrusion into the viewing area of secondary mirror images. There has been no way up until now to control these extraneous images. For instance, oscillating mirrors, often located between the telescope sight line and the rotary prism, can only correct the images on the primary mirror through which the web is sighted. The extraneous images produced on a secondary mirror within the viewing area cannot receive the proper correction. This results in image movement or separation of two partial images which should be superimposed. The extraneous web images produced can lead to costly inspection errors as well as to viewer fatigue and uncertainty, and also restrict the speed at which the target web can move and still be reliably inspected.
One proposed solution to the problem has been to baffle off areas of the target web to prevent the viewing of extraneous images. However, this is undesirable because it necessarily restricts the portion of the moving target web that can be inspected at any given instant. This sometimes results in inspection error. Also, baffling can sometimes introduce confusing interruptions of images in the viewing area.
It is, therefore, an object of the invention to improve rotary prism web systems by providing methods and apparatus which will correct the extraneous image problem without the use of baffles.
It is a further object of the invention to provide methods and apparatus wherein the sweeping scanning angle continuously maintains correspondence to one repeat length of the web so that the viewing image presented to the viewer by the primary and secondary mirrors are coincident.
It is a further ooject of the invention to provide methods and apparatus for the inspection of repeating patterns of designs on a moving web which can operate reliably at moving web speeds of up to at least 1800 feet per minute without any separation of image problem.